The invention is based on a phosphor for light sources and an associated light source in accordance with the preamble of claim 1. It relates in particular to a garnet phosphor which emits in the long-wave range of the visible spectral region and is to be excited by short wavelengths in the visible spectral region. Suitable light sources are in particular a lamp (primarily a fluorescent lamp) or an LED (light-emitting diode), which overall generates white light, for example.
WO 98/05078 has already disclosed a phosphor for light sources and an associated light source. In that document, the phosphor used is a garnet of the structure A3B5O12, the host lattice of which, as first component A, comprises at least one of the rare earths Y, Lu, Sc, La, Gd or Sm. Furthermore, one of the elements Al, Ga or In is used for the second component B. The only activator used is Ce.
A very similar phosphor is known from WO 97/50132. The activator used in that document is either Ce or Tb. While Ce emits in the yellow spectral region, the emission from Tb is in the green spectral region. In both cases, the complimentary color principle (blue-emitting light source and yellow-emitting phosphor) is used to achieve a white luminous color with a semiconductor element.
Finally, EP-A 124 175 describes a fluorescent lamp which, in addition to a mercury fill, contains a plurality of phosphors. These are excited by UV radiation (254 nm) or also by short-wave radiation at 460 nm. Three phosphors are selected in such a way that they add up to form white (color mixture).
The object of the invention is to provide a phosphor in accordance with the preamble of claim 1 which is able to withstand high thermal loads and is eminently suitable for excitation in the short-wave visible spectral region.
This object is achieved through the characterizing features of claim 1. Particularly advantageous configurations are given in the dependent claims.
In detail, according to the invention, a phosphor is proposed for excitation by a radiation source whose emission is in the short-wave optical spectral region. The phosphor has a garnet structure A3B5O12, and it is doped and activated with Ce, the second component B representing at least one of the elements Al and Ga. The first component A contains a rare earth RE selected from the group Y, Sc, Gd, Tb, La and/or Lu, with an amount of at most 5 mol % of A being replaced by praseodymium (Pr). Because of the concentration quenching to be observed with Pr, above all an amount of at most 5 mol % is to be recommended. Particularly favorable is an amount of at most 1 mol %. In this case praseodymium acts as second activator in addition to Ce according to the formula A3B5O12: (Ce, Pr).
Advantageously, the first component A is predominantly (more than 75 mol %) formed by yttrium and/or lutetium, in order to achieve a high efficiency. In addition thereto, it is possible to use amounts of Tb, Sc, Gd and/or La for fine-tuning. Particularly advantageous is the addition of Tb to the component A in small quantities (0.1 to 20 mol %) , since Tb improves the temperature quenching. Good results are furnished, in particular, by a garnet (Y, Tb)3Al5O2: (Ce, Pr).
The phosphor according to the invention can be excited in a wide range of the blue spectral region by radiation in the range from 420 to 490 nm, in particular 430 to 470 nm. A particularly good matching can be achieved to a light source, the peak wavelength of which is in the range 440 to 465 nm.
The phosphor has in particular a garnet structure
(RE1xe2x88x92xxe2x88x92yPrxCey)3(Al, Ga)5O12,
where
RE=Y, Sc, Tb, Gd, La and/or Lu.
The concentration of the two activators should be selected in the following ranges:
0.00005xe2x89xa6xxe2x89xa60.05;
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The second component B contains advantageously both Ga and Al and may additionally contain In.
It has become evident that the addition of Pr to the YAG:Ce host crystal must be accurately dimensioned, since, when the concentration is too high, the luminous efficacy deteriorates significantly, while, when the dimensioning is too low, a marked effect of the red improvement no longer occurs. A good indication for the calculation of the Pr are its lines which occur in the emission spectrum. In particular the amount of the praseodymium and the condition of the host lattice (selection of the components A and B) should be selected in such a way that, essentially, lines of the Pr below 650 nm appear in the emission spectrum, in particular the two lines of the Pr at 609 and 611 nm. The shorter-wave the red component can be selected, the higher the visual useful effect, since the sensitivity of the eyes decreases very strongly towards long wavelengths. A red component which is obtained by an emission at a wavelength above 650 nm, that is, in a range 650 to 700 nm, is therefore considerably less favorable.
In particular the amount of the praseodymium should be less than 0.3 mol % and should, in particular, be selected to be so small that the two lines of the Pr at 609 and 611 nm appear separated from one another in the emission spectrum. In addition, if the Pr amount is suitably dimensioned, the Pr line at 637 nm can also appear in the emission spectrum. Advantageously the amount of the praseodymium should be greater than 0.2 mol % and should be selected to be so high that the Pr line at 637 nm distinctly appears in the emission spectrum. For, as a result of this, an additional contribution in the red spectral region is obtained, in addition to the two other main lines of the Pr at 609 and 611 nm. It should, in particular, amount to at least 10% of the contribution of the other two lines.
The present invention also comprises a light source which primarily emits radiation in the short-wave blue range of the optical spectral region, this radiation being partially or completely converted into longer-wave radiation by means of a phosphor as specified above. In particular, the primary radiation emitted lies in the wavelength range from 420 to 490 nm, in particular 430 to 470 nm. The primary radiation source used is advantageously a blue-emitting light-emitting diode, in particular based on InGaN, in order to produce a white LED. A particularly good matching of phosphor to primary light source is obtained in the range of a peak emission of the LED in the range 440 to 465 nm. This is achieved by combining a blue LED (primary light source) with a phosphor as specified above, which is excited by the radiation from the LED and the emission (secondary radiation source) from which supplements the remaining blue primary LED radiation to form white light. In the case of using a single Pr-containing phosphor, this phosphor should emit mainly in a broad band in the yellow region of the spectrum and should emit additionally in a narrow band in the red region of the spectrum. In the case of using two Pr-containing phosphors, one of the phosphors should emit mainly in a broad band in the yellow region of the spectrum and should emit additionally in a narrow band in the red region of the spectrum (relatively low Pr content), while the second phosphor has an emission curve which is shifted, relatively to this, towards longer wavelength and has a relatively high Pr content (more than 50% higher than the first phosphor). In particular, the concentration of the Pr in the first phosphor should be selected in such a way that only the two Pr lines at 609 and 611 nm appear, while the concentration of the second phosphor may be selected in such a way that the additional line at 637 nm appears also and furnishes a contribution.
A process for producing the phosphor comprises the following process steps:
Comminution of the oxides and adding a flux;
First annealing in forming gas (mixture of H2 and N2);
Milling and screening;
Second annealing.
According to the invention, for light sources whose emission lies in the short-wave blue spectral region, the phosphor used has a garnet structure A3B5O12 and is doped with Ce, the second component B representing at least one of the elements Al and Ga, the first component A containing praseodymium. Surprisingly, it has emerged that praseodymium (Pr) is eminently
0.00005xe2x89xa6xxe2x89xa60.05;
0.01xe2x89xa6yxe2x89xa60.2.
The phosphor absorbs in the range from 400 to 500 nm, preferably between 430 and 470 nm, and can thus be excited by radiation from a blue light source which is in particular the radiation source for a lamp or LED. Good results were achieved with a blue LED with a maximum emission at 420 to 465 nm. When using pure YAG:Ce as base, in particular a range from 440 to 465 nm has proved useful; when using amounts of Lu and Ga, a peak wavelength between 420 and 450 nm is recommended.
This phosphor is particularly suitable for use in a white LED, based on the combination of a blue LED with the garnet phosphor which is excited by absorption of some of the radiation from the blue LED and the emission from which supplements the remaining radiation from the LED to form white light.
A particularly suitable blue LED is a Ga(In)N-LED, although any other way of generating a blue LED with emission in the range from 420 to 490 nm is also suitable. The principal emission range recommended is in particular 430 to 470 nm, since this is when efficiency is highest.
By selecting the type and quantity of rare earths RE, it is possible to finely adjust the position of the absorption bands and the emission bands.
In connection with light-emitting diodes, a range for X which is between 0.0005 and 0.01 is particularly suitable. The preferred range for y is 0.03 to 0.1.
The phosphor according to the invention is also suitable for combination with other phosphors, above suitable as an additional activator in the host lattice (first component A of the garnet), for example under blue excitation in the range from 420 to 490 nm. The standard emission range of the base phosphor of the type YAG:Ce material, the dopant of which is cerium, represents a broad band in the yellow spectral region. Due to the presence of the Pr3+ in the material of the type YAG:Ce3+, additional red light is emitted, with the result that the color locus can be controlled within a wider range and the color reproduction is improved compared to standard YAG:Ce materials. The red emission by Pr3+ is possible because the Pr ion absorbs light which is emitted by the first activator, the Ce ion (above all at the wavelengths 480 to 500 nm), and emits light again. A second transfer mechanism which is surprisingly present is the direct transfer of energy from the Ce ion to the Pr ion. In this case, the Ce ion acts as a sensitizer for the second activator Pr.
The white light is generated, for example, by the combination of an Ga(In)N-LED with the phosphor YAG:Ce. In this case, apart from Pr, it is possible in particular for yttrium to be used as principal constituent of the first component A of the garnet on its own or together with at least one of the rare earths Tb, Gd, Sc, La and/or Lu.
The second component used is at least one of the elements Al or Ga. The second component B may additionally contain In.
In a particularly preferred embodiment, a garnet of the formula A3B5O12: (Ce, Pr) having the stoichiometric structure
(RE1xe2x88x92xxe2x88x92yPrxCey)3(Al, Ga)5O12 is used,
where
RE=Y, Gd, Sc, Tb, La and/or Lu;
all green-emitting phosphors, for example, with a green-emitting phosphor such as thiogallate, in which case above all gallium is under consideration as a constituent of the metallate, as well as, in particular, with other YAG:Ce phosphors, in particular a type which emits more (red) in the long-wave range. However, a particular advantage is that the addition of Pr often makes the use of a further phosphor which, in terms of its centroid, tends to emit more in the red spectral region superfluous.